On 03/26/2013 05:16 PM, Jamie wrote:
jim beam wrote:

On 03/26/2013 05:58 AM, Scott Dorsey wrote:

In article , jim beam
wrote:

On 03/25/2013 05:57 PM, Jamie wrote:

jim beam wrote:

Another way to do this, is to have an inductor on board with the
speed control circuit. You would PWM that inductor in series to a
filter
cap
on the output which will then give you a clean variable DC. THe
inductor
will be doing all variable voltages.


ok, as i understand it, and as i said to scott earlier, this is a
problem because it mungs low speed motor start and low speed torque.


Not really, it's feeding the motor with variable DC from that integrator
stage.


i understand that - and variable voltage is the problem. the
secondary [bordering on primary in some applications] advantage of pwm
is low speed start and torque. if a motor starts at low dc voltage,
not only is the start speed inconsistent, it has little torque. pwm
can start a motor slower and at much higher torque. it's a big deal.


A properly working blower motor does not need extra torque to start at
low RPMs.

Torque is only needed when RPMs increase and mass air flow is
increased, thereby, putting a strain on the motor.

you're right, except that there are more variables. very cold days,
very windy days, blown snow powder, leaves, all kinds of things can mess
with the motor starting at a low speed.


Basic resistor
systems will vary in speed if air pressure isn't constant, and in
most cases it isn't..

When there is no air flow or the flow has been restricted somehow,
there is little to no torque demand, other than mechanical of the blower
blades and those should turn easy, sine bearings and balance permits this.

PWM is just a cheap way of speed control, it does not mean it's
better, in fact in some ways it's not, due to over head in noise..

from what i can see, the /only/ drawback is noise. power efficiency,
controllability, speed consistency, and yes, sometimes price, all are
wins for pwm.



A linear control with feed back will provide the needed torque but
they do tend to run hot when throttled back, because of the resistance
being present between the 12V and the motor terminals. THis is where
PWM comes in a winner but then you need the added cost of noise
reduction engineering.

I can only assume the linear module at least uses a feed back to
maintain output voltage, if it is so cheap that it does not even
do that, then maybe they are trying to emulate a real resistor or
they are just ****ty engineers or tightwads.

well, they're clearly failing at something if they're trying to provide
an engineering solution. if however they're providing a financial
solution with a per-determined failure rate, then they're right on target.


--
fact check required